The accumulation of excess triglyceride in the liver is known as hepatic steatosis (or fatty liver), and is associated with adverse metabolic consequences, including insulin resistance and dyslipidemia. Fatty liver is frequently found in subjects having excessive alcohol intake and subjects having obesity, diabetes, or hyperlipidemia. However, in the absence of excessive alcohol intake (>10 g/day), nonalcoholic fatty liver disease (NAFLD) can develop. NAFLD refers to a wide spectrum of liver diseases that can progress from simple fatty liver (steatosis), to nonalcoholic steatohepatitis (NASH), to cirrhosis (irreversible, advanced scarring of the liver). All of the stages of NAFLD have in common the accumulation of fat (fatty infiltration) in the liver cells (hepatocytes).
The NAFLD spectrum begins with and progress from its simplest stage, called simple fatty liver (steatosis). Simple fatty liver involves the accumulation of fat (triglyceride) in the liver cells with no inflammation (hepatitis) or scarring (fibrosis). The next stage and degree of severity in the NAFLD spectrum is NASH, which involves the accumulation of fat in the liver cells, as well as inflammation of the liver. The inflammatory cells destroy liver cells (hepatocellular necrosis), and NASH ultimately leads to scarring of the liver (fibrosis), followed by irreversible, advanced scarring (cirrhosis). Cirrhosis that is caused by NASH is the last and most severe stage in the NAFLD spectrum.
NAFLD is now the leading cause of chronic liver disease in the United States and there are currently about 1 million patients diagnosed with NASH. Subjects having NASH have a doubled risk of cardiovascular mortality as compared to the general population and NASH is the third most common cause of liver transplantation.
A central element in the regulation of lipid biosynthesis in the human liver is a group of transcription factors termed Sterol Regulatory Element Binding Proteins (SREBPs). There are three SREBP isoforms called SREBP-1a, SREBP-1c and SREBP-2. They are located in the endoplasmatic reticulum (ER) in precursor form (Yokoyama C. et al., Cell 1993, 75:187; Hua X. et al., Proc. Natl. Acad. Sci. 1993, 90:11603) which, in the presence of cholesterol, is bound to cholesterol and two other proteins: SCAP (SREBF chaperone) and Insig1 (Insulin-induced gene 1). When cholesterol levels fall, Insig-1 dissociates from the SREBP-SCAP complex, allowing the complex to migrate to the Golgi apparatus, where SREBP is cleaved by S1P and S2P (site 1/2 protease; Sakai J et al, Mol. Cell. 1998, 2:505; Rawson R. B. et al, Mol. Cell. 1997, 1:47), two enzymes that are activated by SCAP. The cleaved SREBP then migrates to the nucleus and acts as a transcription factor by binding to the SRE (sterol regulatory element) of a number of genes and stimulating their transcription (Briggs M. R. et al., J. Biol. Chem. 1993, 268:14490). Among the genes transcribed are the LDL-Receptor, up-regulation of which leads to increased in-flux of cholesterol from the bloodstream, HMG-CoA reductase, the rate limiting enzyme in de-novo cholesterol synthesis (Anderson et al, Trends Cell Biol 2003, 13:534), as well as a number of genes involved in fatty acid synthesis, e.g., Patatin-Like Phospholipase Domain Containing 3 (PNPLA3), which is a multifunctional enzyme which has both triacylglycerol lipase (triglyceride hydrolysis) and acylglycerol O-acyltransferase activities (triglyceride synthesis).
Since SCAP-binding is a prerequisite for the transport and activation of all three SREBP isoforms, inhibition of the expression and/or activity of SCAP with an agent that can selectively and efficiently attenuate the body's own lipid biosynthesis, e.g. by inhibiting SCAP, and thereby SREBP activity, would be useful for treating pathological processes mediated directly or indirectly by SCAP expression.
Currently, treatments for NAFLD are directed towards weight loss and treatment of any secondary conditions, such as insulin resistance or dyslipidemia. However, to date, no pharmacologic treatments for NAFLD have been approved. Therefore, there is a need for therapies for subjects suffering from NAFLD, e.g., steatosis and NASH.